* Joseph Schuchart <joseph.schuch...@tu-dresden.de> wrote:
> > Just a quick side note, while I realize that you are
> > (rightfully!) concerned about correctness primarily, if that loop
> > over MAX_NR_CPUS executes often enough then this might hurt
> > performance:
> >
> > perf.h:#define MAX_NR_CPUS 256
> >
> > So it might be better to maintain a rolling min_max_timestamp in
> > this place:
> >
> > + os->max_timestamps[sample->cpu] = timestamp;
> >
> > ?
> >
> > If done that way then AFAICS we could even eliminate the
> > ->max_timestamps[NR_CPUS] array.
>
> I can understand your performance concerns. However, I am not
> sure how we can determine the minimal max_timestamp of all cpus
> without storing the information on a per-cpu basis first.
> Accumulating it on the fly would only lead to a global
> max_timestamp. [...]
Ok. So this:
+static inline void set_next_flush(struct perf_session *session)
+{
+ int i;
+ u64 min_max_timestamp = session->ordered_samples.max_timestamps[0];
+ for (i = 1; i < MAX_NR_CPUS; i++) {
+ if (min_max_timestamp >
session->ordered_samples.max_timestamps[i])
+ min_max_timestamp =
session->ordered_samples.max_timestamps[i];
+ }
+ session->ordered_samples.next_flush = min_max_timestamp;
+}
which should IMHO be written in a bit clearer form as:
static inline void set_next_flush(struct perf_session *session)
{
u64 *timestamps = session->ordered_samples.max_timestamps;
u64 min_timestamp = timestamps[0];
int i;
for (i = 1; i < MAX_NR_CPUS; i++) {
if (min_timestamp > timestamps[i])
min_timestamp = timestamps[i];
}
session->ordered_samples.next_flush = min_timestamp;
}
calculates the minimum of the max_timestamps[] array, right?
Now, the max_timestamps[] array gets modified only in a single
place, from the sample timestamps, via:
os->max_timestamps[sample->cpu] = timestamp;
My suggestion was an identity transformation: to calculate the
minimum of the array when the max_timestamps[] array is modified.
A new minimum happens if the freshly written value is smaller
than the current minimum.
I.e. the max_timestamps[] array itself is redundant, and we just
have to update a rolling minimum - which is a (session-) global
minimum - which is equivalent to the more complex minimum
calculation in your patch.
What am I missing?
Thanks,
Ingo
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